Direct current converters or DC power supplies are conventionally powered from one or two sources, usually either regular mains or factory power. In a single-source system, of course, each module powered by the power supply receives power from a single source. The powered modules are simply connected in parallel via a single power bus system. In a conventional "fail-safe" or double-source system, each of the powered modules is connected in parallel along a first bus to a first source and also along a redundant second bus to a second source; each module receives power from either of the two sources. This technique provides protection in the case of a single supply failure (or supply source failure when the supplies are fed power from separate sources) and in the case of any individual module presenting an open circuit to the supply bus. These are the more common failure mechanisms; the prior art methods usually work well as simple redundant supply techniques.
However, when a module fails such that a short circuit is presented to the bus or when a bus or power supply fails such that the short circuit is presented to all modules along the bus, the reliability of the redundant supplies fails and continued supply of power to the modules is lost. Failure of both the first and second busses supplying a given set of modules results in failure of all the modules in that set. The present invention is directed to solving these problems.
The availability of power to operate the equipment in a process control system, like many other systems, is dependent on the availability of power. Availability is a function of reliability and mean-time-to-repair. Often, when a module or power supply fails it is difficult to repair/replace it without turning off power to the entire system or at least disconnecting a group of modules. It is preferable to restore the power supply with minimal loss of power to the system or subsystem. This is called `hot` repair or replacement, and is facilitated by the present invention. `Hot` repair was difficult or impossible to safely achieve with many prior art redundant power supply techniques. The present invention facilitates rapid repair by enabling replacement of failed modules or power supply units without disrupting the entire system or subsystem in which the power failure occured.
Prior art redundant mains (also called `factory`) power frequently consisted of merely switching between two sources of AC power. This method may involve a `dropout` of AC power, which if of sufficiently long duration is harmful to solid-state systems and causes loss of memory in power-dependent memory devices. An embodiment of the present invention extends the redundant technique to a supply architecture in which line voltage dropout is minimized or eliminated, and in combination with the power distribution technique disclosed provides fault tolerance which provides both fully redundant power and power distribution using the same set of modules required for non-redundant power.